1. Field of the Invention
The present invention relates to a reflective (catoptric) imaging optical system, an exposure apparatus, and a method for producing a device. In particular, the present invention relates to a reflective imaging optical system preferably useable for an exposure apparatus which uses the EUV light (EUV light beam) and transfers, onto a photosensitive substrate, a circuit pattern on a mask for example in accordance with the mirror projection system.
2. Description of the Related Art
Attention is directed to an EUVL (Extreme UltraViolet Lithography) exposure apparatus which uses the EUV (Extreme UltraViolet) light having a wavelength of, for example, about 5 to 40 nm as an exposure apparatus to be used for producing semiconductor elements, etc. In a case that the EUV light is used as the exposure light (exposure light beam), any usable transmissive optical material and any useable dioptric optical material are absent. Therefore, a reflection type mask is used, and a reflective optical system (optical system constructed of only reflecting members) is used as a projection optical system.
Conventionally, it has been suggested that a reflective optical system, which has an incident pupil disposed on a side opposite to the optical system with an object plane intervening therebetween, is used as a reflective imaging optical system applicable to a projection optical system of an EUVL exposure apparatus, in place of a reflective optical system which has an incident pupil disposed on a side of the optical system with an object plane intervening therebetween (see, for example, U.S. Pat. No. 6,781,671). In the following description of this specification, the “reflective imaging optical system having the incident pupil disposed on the side of the optical system with the object plane intervening therebetween” is referred to as “reflective imaging optical system of the near pupil type”, and the “reflective imaging optical system having the incident pupil disposed on the side opposite to the optical system with the object plane intervening therebetween” is referred to as “reflective imaging optical system of the far pupil type”. In other word, the former is also referred to as “reflective imaging optical system of the normal pupil type” and the latter is also referred to as “reflective imaging optical system of the opposite pupil type”.
When the reflective imaging optical system of the near pupil type is adopted as the projection optical system of the EUVL exposure apparatus, it is necessary that a condenser optical system is arranged in an optical path between a mask and an optical integrator in an illumination optical system. Therefore, the optical efficiency (light efficiency) is lowered due to the increase in the number of mirrors. On the contrary, when the reflective imaging optical system of the far pupil type is adopted, it is unnecessary to arrange any condenser optical system. Therefore, it is possible to enhance the optical efficiency owing to the reduction of the number of mirrors.
However, with respect to the reflective imaging optical system of the far pupil type disclosed in the U.S. Pat. No. 6,781,671, a distance which is provided along the optical axis between the incident pupil and the object plane (hereinafter simply referred to as “incident pupil distance” as well) is relatively short. Therefore, any overlay error (overlap error) tends to arise in relation to respective illumination fields formed on the mask by respective light fluxes subjected to the wavefront division by a first fly's eye optical system of the optical integrator. Consequently, the light amount loss tends to arise. If only the incident pupil distance is simply lengthened in the reflective imaging optical system of the far pupil type, then the distance from the optical axis of the effective field on the object plane of the reflective imaging optical system is excessively increased, and it is difficult to satisfactorily correct the aberration.
The present invention has been made taking the foregoing problems into consideration, an object of which is to provide a reflective imaging optical system of the far pupil type as a reflective optical system applicable, for example, to an exposure apparatus using the EUV light wherein the incident pupil distance is secured to be relatively large and the aberration is satisfactorily corrected. Another object of the present invention is to perform the projection exposure at a high resolution while securing a large resolving power by using, for example, the EUV light as an exposure light, with the application of the reflective imaging optical system of the far pupil type of the present invention to a projection optical system of an exposure apparatus.